(1) Field of the Invention
The present invention relates to salt water desalination, and more particularly to an apparatus and process for desalination of salt water with the production of purified water and high value chemicals.
(2) Description of the Related Art
As population grows, the strain on the world's fresh water supplies will increase. By 2025, about 2.7 billion people, nearly one-third of the projected population, will live in regions facing severe water scarcity. Many prosperous and fast growing regions—the American Southwest, Florida, Asia, the Middle East—have inadequate freshwater supplies. But other factors such as a pleasant climate and mineral resources, job growth and rising incomes cause population growth nevertheless. The water needs of municipalities, industry, and citizens must be met, even as the difficulty and cost of developing new water resources increases.
Desalination has become a more popular option in regions where there is abundant water that is unsuitable for use due to high salinity, and there are opportunities for desalination plants that utilize thermal, electrical or mechanical energy to separate the water from the salts. The choice of the desalination process depends on many factors including salinity levels in the raw water, quantities of water needed, and the form of available energy.
Reverse osmosis (RO) is generally accepted as the most economical and energy-efficient method for desalination of highly saline water. See, e.g., Al-Sofi, Desalination, 135:121–139 (2001). Modern RO membranes have such high salt rejection that they are capable of producing potable water, <500 ppm salinity, from seawater (nominally 35,000 ppm salinity) in a single pass through the membrane. Furthermore, some modern RO systems are capable of achieving up to 50% recovery of fresh water from seawater. With 50% recovery, the salinity of the concentrated brine increases to about 70,000 ppm. Disposal of such brines presents significant costs and challenges for the desalination industry, which result in longer start-up lead times and higher cost of water. Brine disposal to surface waters in the United States requires NPDES permits that prevent construction in certain high-demand areas. There are three basic ways to deal with brines from seawater desalination—discharge to the sea, deep well injection, and zero liquid discharge systems. The discharge of brines back into the sea can affect the organisms in the discharge area.
Evaporation and electrodialysis (ED), which are proven processes for seawater desalination, can make a brine of considerably higher concentration than can be recovered from RO, but these processes consume more energy than RO in seawater desalination.
One problem that is of concern in many desalination processes is the formation of calcium sulfate scale. In evaporation processes the high temperature at the heat exchange surfaces causes local supersaturation due to reduced solubility of calcium sulfate at elevated temperatures, even when the bulk solution is not saturated. In processes utilizing RO and nanofiltration (NF), conditions of supersaturation can exist at the membrane surface due to buildup of ion concentrations in the boundary layer. Even brackish groundwater often has enough calcium and sulfate ions to limit the amount of fresh water that can be recovered by desalination.
Seawater has many valuable constituents, but their value can only be realized if they can be recovered economically. There are ways to recover many of these valuable seawater constituents, but the economics of the recovery are often dismal because of the low concentrations of those constituents, and due to interference by other constituents of seawater.
One valuable component of seawater is sodium chloride (NaCl). Japan, for example, has no natural salt deposits, and land is too expensive there to allow the use of evaporation ponds for salt manufacture. For several decades Japan has relied on ED to recover table salt from seawater. The seawater is filtered and pumped at low velocity in a single pass through the desalting compartments of very large ED stacks. The voltage applied across membranes and solution compartments forces Na+ ions through the cation permeable membrane on one side of the compartment and Cl− ions through the anion permeable membrane on the other side of the compartment. The Mg++ ions, second most abundant cations in seawater, also migrate in the electric field, but Mg++ passage through the cation permeable membrane is hindered by a special coating on the membrane surface. The passage of SO4= ions is hindered by a coating on the anion permeable membrane. Thus the purity of the NaCl in the brine recovered by ED is substantially higher than the purity of brine prepared by evaporation of raw seawater. After concentration to 20% by ED, the brine is evaporated to dryness with the byproduct heat from the power plant used to generate the electricity for the ED.
Seawater is also used as the feedstock for the production of magnesium and bromine compounds. A commercial method for recovering Mg++ is to add a base (usually lime) to seawater in order to precipitate Mg(OH)2. One disadvantage that the recovery of magnesium from seawater has in comparison with magnesium recovery from magnesite is the low concentration of magnesium in the seawater. If the magnesium content of the brine feed could be increased at a reasonable cost, the production costs for magnesium would be reduced. Accordingly, this would allow manufacturers using seawater as a feedstock to compete more effectively with magnesium producers who use magnesite. Moreover, this would help alleviate the environmental damage associated with magnesite mining operations as well as the generation of the large amount of carbon dioxide incident to the processing of magnesite.
Disposal of the concentrated brine from RO plants is a major concern. The presence of dissolved salts adds to the density of water. The specific gravity (at 20° C.) of seawater (3.5% salts) is about 1.0263, and the specific gravity of high-yield RO reject (7.0% salts) is about 1.0528. If this dense RO reject were to be injected directly into the sea, it would accumulate at the bottom with a devastating effect on bottom-dwelling organisms. The greatest environmental concern associated with brine discharge to surface water relates to the potential harm that concentrate disposal may pose to bottom-dwelling organisms in the discharge area. Following the guideline that a 1000-ppm change in the salinity can be tolerated by most organisms, a given volume of 70,000 ppm brine would require dilution with 34 volumes of seawater. In some cases, that dilution can be achieved by combining the brine with another outflow such as cooling water from a power plant; otherwise an underwater structure is needed to disperse the brine. Such underwater structures are disruptive to the sea bottom, require inspection and maintenance, and are subject to damage by fishing nets, anchors or natural movements at the sea bottom.
The cost of brine disposal to the sea will vary widely depending upon site-specific circumstances. The cost of pipelines into the deep ocean, where the effects of high-salinity brine disposal are more-likely to be negligible, increase exponentially with depth. Coastal locations that are on sheltered bays or that are near estuaries, protected wetlands, and other sensitive ecosystems, are more likely to have trouble disposing of RO concentrate. Concentrate disposal problems rule out many otherwise suitable locations for industrial and municipal reverse osmosis facilities for treatment of seawater (SWRO) or brackish water (BWRO). These concentrate-disposal-constrained sites represent an important potential area for the application of Zero Liquid Discharge (ZLD) desalination. However, the high cost of commercially available ZLD technology (brine concentrators and crystallizers), and the limitations of experimental technologies such as solar ponds and dewvaporation have limited their use in SWRO and BWRO.
Other than return to the sea, the alternatives for disposal of brines from desalination plants are limited. Evaporation ponds are generally undesirable and expensive due to the cost of land. Moreover, they are useful only in climates where evaporation rates exceed rainfall. Deep well disposal is often used for hazardous wastes, and it has been used for desalination brines in Florida, but capital costs are on the order of $1 per gpd of desalination capacity. Furthermore, the applicability of deep well injection for large desalination plants is questionable because of the sheer volume of the brine and the possibility of contamination of groundwater.
Zero Liquid Discharge (ZLD) systems are widely used in other industrial situations where liquid wastes cannot be discharged. These systems usually include evaporative brine concentration followed by crystallization or spray drying to recover solids. Conventional ZLD technology involves a thermal brine concentrator and crystallizer. This technology can be used to separate SWRO concentrate into freshwater and dry salt. However, the capital costs and electrical consumption, ˜$6000–$9000 per cubic meter of daily capacity and ˜30 kWh per cubic meter of freshwater produced, is so high that it has not been used to achieve “zero-discharge” SWRO.
Water removal from dilute brines is usually accomplished by vapor compression or high-efficiency multiple-effect evaporators. The condensate usually has less than 10 ppm of total dissolved solids (TDS). The heat for evaporating water from saturated brines is usually provided by steam, but, even with the efficiencies of vapor recompression, the capital and operating costs of existing ZLD processes are substantial.
In contrast to the pressure-driven RO process, transport in ED is driven by electric potential. The pressure required to push water through an RO membrane is greater than the osmotic pressure, which is roughly proportional to the concentration of dissolved salts, so the necessary pressure increases as fresh water recovery increases. The limitation of the brine concentration in RO is thus determined by the practical limits of materials of construction and the energy required to pump the water to these high pressures. Since ED is electrically driven, osmotic pressures are almost inconsequential. The maximum concentration of salts in the ED brine is limited by two factors—the solubility of the salts and the amount of water that moves through the membranes along with the salt ions.
Special membranes developed for manufacture of salt from seawater in Japan have the ability to selectively pass univalent ions (Na+, K+, Cl−, Br−) while rejecting multivalent ions (Ca++, Mg++, SO4=). Use of these univalent-selective membranes is beneficial because the NaCl purity in the ED brine is higher and because the concentration of sparingly soluble CaSO4 in the brine is substantially lower that would be the case if ordinary ion-exchange membranes were used.
In contrast with RO, the energy for salt transport by ED decreases with concentration. Desalination by RO requires about 5 kWh to produce a cubic meter of potable water compared to about 7.4 kWh for ED, which corresponds to 215 kWh per ton of salt removed by ED during desalination. However, electrical energy consumption for ED in salt manufacture is only about 150 kWh per ton of salt produced. This difference is due to the fact that the diluate (the solution from which salt has been removed in an ED unit) is more concentrated (thus more conductive) when the ED unit is being used for salt recovery than when it is used in desalination.
Despite the progress that has been made in salt water desalination, it would be useful to provide processes and equipment that efficiently recover purified water from salt water, and, in particular, to provide processes and equipment that could efficiently recover other valuable chemicals, such as high-purity sodium chloride, magnesium and bromine, in addition to purified water.